This invention relates to MOS-image sensors, particularly line-type or area-type sensors having a plurality of signal-integration photo-sensitive elements sequentially read out by controlling the operation of signal readout gate switches in response to a signal readout address arrangement.
In so-called self-scanning types of solid state image pickup devices, such as MOS-sensors, CCD-image sensors, and BBD-image sensors, the light information impinging thereon is integrated over a period of time and then read out. It is known that as the integration time is varied the response characteristic to incident light, that is the dynamic range, is extended to provide a scanning output (image pickup output level) proper for the incident light which may change over a wide range. Various methods of controlling this integration time have been proposed. For example, according to a typical proposal, the frequency of drive clock pulses which control the operation of the solid state image pickup device is varied to adjust the integration time. This is disclosed, for example, in Japanese Open Patent No. Sho 51-46019. According to another proposal, particularly suitable for CCD-image sensors or BBD-image sensors, the width of a carrier storage control pulse to be fed to the light receiving portions is made variable and the change of width adjusts the integration period, in this instance, the carrier storage period. This is disclosed in Japanese Open Patent Nos. Sho 50-62518 and Sho 50-92635.
These conventional methods of adjusting the integration time have a number of disadvantages. For example, varying the clock frequency changes the scanning frequency, and this automatically changes the output time of the scanning signal. Therefore, when used in television cameras, this arrangement fails to keep the output time of the scanning signal constant, and is hence, unsuitable for this purpose. Furthermore, reduction of the integration period involves increasing the scanning clock frequency, and because as is generally known, an increase in the scanning clock frequency causes deterioration of the efficiency of signal readout, it is impossible to increase the frequency to any great extent. Therefore, the adjustable time range is largely limited, particularly to shorter times. Another disadvantages arising from increasing the scanning clock frequency is that the amount of electrical energy consumed increases therewith. On the other hand, although making the width of the carrier storage pulse variable does eliminate some of these drawbacks, this method is applicable only to charge transfer type image sensors such as CCD-image sensors and BBD-image sensors. Carrier storage pulse width variation is entirely inapplicable to MOS-type image sensors.